This new post from the WPEngineer site gives you a few tips on using the event scheduling (cron) that the default WordPress installation already has built in.

WordPress has its own cron to automatically and scheduled run certain themes. Therefore WordPress provides several functions to use the cron.

In their example they show how to define the task to be executed hourly (sending an email), removing it from the task list and how to define a few more time fields (custom intervals). You can find out more about these event scheduling functions on this page of the WordPress codex site